I made an open-source tool in Go, named gogg, to download and back up your GOG.com game library.

It's cross-platform and has features like:

- A scriptable CLI and easy-to use GUI

- Multi-threaded and resumable downloads

- Filters for platform, language, DLCs, etc.

- File verification with hashes and total size calculation

You can find the project on GitHub: https://github.com/habedi/gogg